1. Field of the Invention
The invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program therefor that perform intercommunication between multiple radio stations as in the case of a LAN (local area network). More specifically, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program therefor that implement broadband wireless communication in communication environments such as home.
More specifically, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program therefor that implement transmission capacity enhancement through communication (MIMO (multi-input multi-output) communication) wherein multiple logical channels are formed by using spatial multiplexing between a transmitter and a transmitter in a pair, the transmitter having multiple antennas and the receiver having multiple antennas. More specifically, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program therefor that perform communication operation with high transmission efficiency by using an increased communication capacity acquired by performing optimal allocation of transmission powers to multiple logical channels obtainable by spatial multiplexing.
2. Description of the Related Art
Sharing such as information resource sharing and device resource sharing can be efficiently implemented through computer networking across a network represented by, for example, a LAN (local area network). As a system relieving users from LAN wiring in accordance with a hardwire method having been used, attention is now drawn to wireless LANs. With a wireless LAN, in an operation spacing such as an office work, since most of hardwiring can be omitted, communication terminals, such as personal computers (PCs), can be relatively easily moved.
Under recent commercial market circumstances wherein wireless LAN systems with enhanced communication speed become available at reduced prices, demands therefor are significantly increasing. Particularly, a personal area network (PAN) has been studied and taken into consideration for introduction to perform information communication by forming a small-scale wireless network between multiple electronic devices existing around individual user bodies. Different wireless communication systems and wireless communication apparatuses are regulated with the use of frequency bands, such as a 2.4 GHz band and a 5 GHz band, for which governmental licenses are not necessary.
Standards regarding wireless networks include, but not limited to, IEEE (The Institute of Electrical and Electronics Engineering) 802.11 (see Non-Patent Document 1(*), for example), HiperLAN/2 (see Non-Patent Document 2(*) or 3(*), for example), IEEE 302.15.3, and Bluetooth communication. Regarding the IEEE 802.11 standard, there are extended standards such as IEEE 802.11a (see Non-Patent Document 4(*), for example), 11b, and 11g.
The IEEE 802.11a standard supports a modulation method that achieves a maximum transmission speed of 54 Mbps. However, radio standard specifications allowing even higher bit rates are sought. For example, according to IEEE 802.11n, standards on next-generation wireless (or, radio) LANs are established with an aim for development of wireless LAN techniques having a high processing rate exceeding 100 MBPS in execution throughput.
As one of the techniques of enhancing wireless transmission speeds, attention has been and is focused on MIMO (multi-input multi-output) communication scheme. The MIMO communication scheme is a scheme that includes pluralities of antenna devices on on individual sides of a transmitter and a receiver to implement spatially multiplexed transmission lines (which hereinbelow will be alternatively referred to as “MIMO channels”), thereby implementing the transmission capacity enhancement and, consequently, accomplishing the transmission speed enhancement. The MIMO communication uses the spatial multiplexing, so that it exhibits high frequency use efficiency.
The MIMO communication scheme is a communication scheme that uses channel characteristics in the following manner. In the transmitter, transmission data are allocated and sent to the multiple antennas and are transmitted by using multiple virtual or logical MIMO channels. In the receiver, reception data is received through signal processing from signals received through the plurality of antennas. As such, the scheme is different from a simple transmission/reception adaptive array.
FIG. 8 is a conceptual representation of a MIMO communication system. With reference to the figure, multiple antennas are deployed individually at a transmitter and a receiver. On the side of the transmitter, multiple items of transmission data are multiplexed through space-time coding (which alternatively will be expressed as “space-time coded,” hereafter), allocated to M (=positive integer) antennas and sent therefrom to multiple MIMO channels. On the side of the receiver, reception signals received by N (=positive integer) antennas through the channels can be acquired through space-time decoding (which alternatively will be termed “space-time decoded,” hereafter). A channel model in this case is configured of a radio environment (transfer function) around the transmitter, a channel space structure (transfer function), and a radio environment (transfer function) around the receiver. While crosstalk occurs during the multiplexing of the signals transmitted from the individual antennas, individual signals multiplexed by signal processing of the receiver can be received in correct forms without crosstalk.
Various types of configuration method for MIMO communication have been proposed. In this case, a big problem in forming the configuration is how to cause channel information to be communicable between a transmitter and a receiver in correspondence to antenna configurations.
An easy method for communication of the channel information is to transmit preliminarily known information (preamble information) from the transmitter to only the receiver. In this case, the transmitter and the receiver spatial perform spatial multiplex transmission independently of each other. This is called an “open-loop MIMO communication scheme (system).” In addition, as an extended form of the scheme, there are known closed-loop MIMO communication schemes wherein preamble information is fed back from the receiver to also the transmitter, thereby to create ideal spatial orthogonal channels between the transmitter the receiver.
Open-loop MIMO communication schemes include, for example, a V-BLAST (Vertical Bell Laboratories Layered Space Time) scheme (see Patent Document 1(*), for example). On the side of transmitter, a specific antenna matrix is not provided, but signals are simply multiplexed and sent to individual antennas. That is, a feedback procedure to obtain an antenna matrix is totally omitted. Before transmitting the multiplexed signal, the transmitter time-divisional interleaves a training signal in units of the antenna for being used for channel estimation on the side of the receiver. In response, in the receiver, channel estimation is performed in a channel estimation section thereby to calculate a channel matrix H corresponding to individual antenna pairs. Then, zero-forcing and canceling are well combined, whereby the degree of antenna freedom caused by canceling is used to improve the SN (signal to noise) ratio (“SNR,” hereafter) and to thereby improve decoding accuracy.
In addition, as an ideal type of closed-loop MIMO communication, an SVD-MIMO scheme using propagation-path SVD (SVD: singular value decomposition) is known (see Non-Patent Document 5(*), for example).
FIG. 9 is a conceptual view showing an SVD-MIMO communication system. In SVD-MIMO communication, a numeric matrix formed of elements of channel information corresponding to individual antennas, namely, the channel matrix H is decomposed by the singular value decomposition to obtain UDVH, whereby V is given as an transmission antenna weight matrix on the side of the transmitter, and UH is given as the antenna weight factor matrix on the side of the receiver. In this manner, the individual MIMO channels are represented as a diagonal matrix D having the square roots of individual eigenvalues λi as diagonal elements, whereby signals completely free of crosstalk can be multiplexed and transmitted. In this case, a plurality of logically mutually-independent transmission paths spatial-divided, that is, spatial-orthogonal multiplexed can be implemented on both sides of the transmitter and the receiver.
According to the SVD-MIMO communication scheme, a logically maximum communication capacity can be accomplished. As such, with the transmitter and the receiver each having two antennas, a double transmission capacity at maximum can be acquired.
A mechanism of the SVD-MIMO communication scheme will now be described in detail here. Where the number of antennas of the transmitter is M, a transmission signal x is denoted by an M×1 vector; and where the number of antennas of the receiver is N, a transmission signal y is denoted by an N×1 vector. In this case, the channel characteristics are denoted by an N×M numeric matrix, that is, the channel matrix H. An element hij of the channel matrix H is a transfer function from a j-th transmission antenna to an i-th reception antenna. The reception signal vector y is represented by equation (1) given below, wherein the channel matrix H is multiplied by the transmission signal vector, and a noise vector n is added.y=Hx+n  (1)
As described above, when the channel matrix H is decomposed by the singular value decomposition, the result is represented by equation (2) given below.H=UDVH  (2)
A transmission antenna weight matrix V on the side of the transmitter side and a reception antenna weight matrix U on the side of the receiver are, respectively, unitary matrixes satisfying equations (3) and (4) given below.UHU=I  (3)VHV=I  (4)
More specifically, an arrangement of normalized eigenvectors of HHH is a reception antenna weight matrix UH on the side of the receiver, and an arrangement of normalized eigenvectors of HHH is a transmission antenna weight matrix V on the side of the transmitter. D denotes a diagonal matrix having square roots of eigenvalues of either HHH or HHH as diagonal components. The matrix has a size represented by the smaller of the number of transmission antennas M and the number of reception antennas N, therefore forming a square matrix or diagonal matrix having the size of min[M,N].
                    D        =                  [                                                                                          λ                    1                                                                              ⋯                                                                                                                          0                                                                    ⋮                                                                                  λ                    2                                                                                                                                                                                                                                                                                                                                                                                                                              ⋱                                                                                                                                                  0                                                                                                                                                                                                                                              λ                                          min                      ⁡                                              (                                                  M                          ,                          N                                                )                                                                                                                          ]                                    (        5        )            
Hereinabove, although the singular value decomposition has been described using the real numbers, cautions exists on singular value decomposition when being extended to the magnitude of each of the complex numbers. Whereas U and V each represent the matrix formed of eigenvectors, even when the eigenvectors are steered or normalized so that the norm thereof becomes equal to one, the eigenvectors are not singularized, but there are an infinite number of eigenvectors having phases different from one another. Even a case takes place in which the above equation (2) is not satisfied depending on the phase relation between U and V. This is because while U and V are individually correct, only the individual phases arbitrarily rotate. To completely match the phases, V is acquired as the eigenvectors of HHH in the regular procedure. Concurrently, U is acquired such that V is multiplied from the right by both sides of the above equation (2), as in equation (6) given below.HV=UDVHV=UDI=UD U=HVD−1  (6)
When weighting is performed using the transmission antenna weight matrix V on the side of the transmitter and when reception is performed by performing weighting with the reception antenna weight matrix UH on the side of the receiver, since U and V are individually the unitary matrixes (U=N×min[M,N]; V=M×min[M,N]), V is represented by equation (7) given below.
                              y          =                                                                      U                  H                                ⁢                HVx                            +                                                U                  H                                ⁢                n                                      ⁢                                                  ⁢                                                  =                                                                                                      U                      H                                        ⁡                                          (                                              UDV                        H                                            )                                                        ⁢                  Vx                                +                                                      U                    H                                    ⁢                  n                                            ⁢                                                          ⁢                                                          =                                                                                          (                                                                        U                          H                                                ⁢                        U                                            )                                        ⁢                                          D                      ⁡                                              (                                                                              V                            H                                                    ⁢                          V                                                )                                                              ⁢                    x                                    +                                                            U                      H                                        ⁢                    n                                                  ⁢                                                                  ⁢                                                                  =                                  IDIx                  +                                                            U                      H                                        ⁢                    n                                                                                      ⁢                                  ⁢                  y          =                      Dx            +                                          U                H                            ⁢              n                                                          (        7        )            
The reception signal y and the transmission signal x are, respectively, not vectors determined by the number of transmission antennas and the number of reception antennas, but are (min[M,N]×1) vectors.
Since D is the diagonal matrix, individual transmission signals can be received without crosstalk being caused. In addition, since the amplitudes of the individual mutually-independent MIMO channels are proportional to the square roots of the eigenvalues λ, power levels of the individual MIMO channels are proportional to λ.
Also in regard to the noise components n, the column of U is an eigenvector of which the norm is normalized to one, so that UHn is not of a nature that varies the noise power thereof. In regard to the size, UHn becomes a (min[M,N]) vector, so that it has the same size as y and x.
As described above, in the SVD-MIMO communication, although at the same frequency and the same time, a plurality of logically mutually-independent crosstalk-free MIMO channels can be acquired. That is, by using the same frequency at the same time, multiple items of data can be transmitted through wireless communication, and the transmission speed enhancement can be implemented.
Generally, the number of MIMO channels obtainable in the SVD-MIMO communication system corresponds to the smaller of the number of transmission antennas M and the number of reception antennas N, that is, min[M,N]. The transmission antenna weightmatrix V on the side of the transmitter is configured of transmission vectors Vi (V=“V1, V2, . . . , Vmin[M,N]”) corresponding to the number of MIMO channels. In addition, the number of elements of the individual transmission vectors Vi corresponds to the number of transmission antennas M.
Generally, it is known that even more ideal information transmission can be implemented with the closed-loop MIMO scheme represented by the SVD-MIMO scheme in such a manner that the transmission path information is taken into account thereby to optimize the calculation of optimal antenna weight factors, coding rates that are to be provided to bit streams of the individual antennas, and modulation techniques.
In addition, to adopt the closed-loop MIMO scheme as a real system, other problems arise. In the event that channel variations increase in association with the movement of the transmitter and the receiver, there occurs an increase in frequency of necessary feedback from the receiver to the transmitter increases. Further, in the SVD-MIMO communication scheme, it is not easy to perform the operation of singular value decomposition in real time, and a setup procedure for preliminarily notification of V or UH to a destination is necessary.
The following will describe a case incorporating consideration regarding the amount of information of the transmission antenna weight matrix V on the side of the transmitter (“V transmission antenna weight factor matrix,” hereafter) by reference to, as an example, the IEEE 802.11a system, which is a LAN system usable as an adaptation object of the SVD-MIMO communication, more specifically, a 5-GHz band OFDM (orthogonal frequency division multiplexing) communication scheme. If the numbers of transmission/reception antenna elements are each three, the transmission antenna weight matrix V is a 3×3 matrix, and the number of elements thereof is nine. When the matrix is represented by real numbers and complex numbers having an accuracy of 10 bits per element and it is necessary for 52 carriers, 9360 bits (=9 (number of elements of the matrix)×2 (real part and imaginary part of the complex number))×10 (bits)×52 (number of OFDM sub-carriers) have to be fed back to the transmitter from the receiver.
The following will describes matters that should be considered when configuring an actual SVD-MIMO communication system.
According to a basic configuration of the SVD-MIMO communication scheme, in the receiver, the acquired channel matrix H is decomposed by the singular value decomposition, the reception weight matrix UH and the transmission weight matrix V is acquired, and V is fed back to the transmitter. V is used as a transmission weight on the side of the transmitter.
However, in an event where, for example, the amount of information of V is large and hence the information of V is reduced and transmitted, the inter-MIMO-channel orthogonal state is collapsed because of a difference from the original information of the matrix, and crosstalk is generated thereby.
As such, ordinarily, after the transmission antenna weight matrix V received by the receiver has been sent to the transmitter, the transmitter weights a reference signal by using V to thereby transmit the signal, and the receiver re-acquires the channel matrix. When the channel matrix is H, the receiver can acquire a channel matrix HV from the reference signal weighted with V and transmitted.
On the side of the receiver, an inverse matrix of HV is acquired and used as a reception weight. Since H=UDVH, HV is represented by equation (8) given below.
                                              ⁢                  HV          =                                                    UDV                H                            ⁢              V                        ⁢                                                  ⁢                                                  =                                                            UD                  ⁢                                                                          (                  HV                  )                                -                            =                                                                    (                    UD                    )                                    -                                =                                                                            D                      -                                        ⁢                                          U                      -                                                        =                                                            D                      -                                        ⁢                                          U                      H                                                                                                                              (        8        )            
The above is simply acquired such that after the same UH as that of the ordinary SVD-MIMO is used as the reception weight, a constant obtainable from a respective diagonal element λi of the diagonal matrix D is multiplied by the respective separated MIMO channel.
Thus, in the configuration, V is used as the transmission weight on the side of the transmitter, and the inverse matrix of HV is used as a reception weight on the side of the receiver. This configuration has the same performance as the performance of the ordinary SVD-MIMO, wherein no mismatch in V exists between the sides of the transmitter and the receiver. Accordingly, the configuration can be employed in a practical application.
Notes (*)    Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-84324 (or, 1991-84324)    Non-Patent Document 1: International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications    Non-Patent Document 2: ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions    Non-Patent Document 3: ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer    Non-Patent Document 4: Supplement to IEEE Standard for Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band    Non-Patent Document 5: http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf (As of Oct. 24, 2003)
As described above, according to the SVD-MIMO communication scheme, a plurality of logical mutually-independent communication paths (MIMO channels) not having interference (crosstalk) with one another even at the same frequency and the same time can be acquired. In more particular, the communication scheme enables transmission of multiple items of data thorough wireless communication by using the same frequency at the same time. Thereby, transmission speed enhancement can be implemented.
According to the SVD-MIMO communication scheme, weighting is performed by using the transmission antenna weight matrix V on the side of the transmitter, and weighting is performed by using the UH antenna weight factor matrix on the side of the receiver, so that the reception signal y relative to the transmission signal x is expressed as in the above equation (7). In addition, since D is the diagonal matrix, individual transmission signals can be received without crosstalk. Further, since the amplitudes of the individual mutually-independent MIMO channels are proportional to the square roots of the eigenvalues λ, power levels of the individual MIMO channels are proportional to λ.
This implies that when the power is equally allocated to the transmission antennas on the side of the transmitter, each ratio thereof is the same as each of the ratios of the powers of the mutually independent MIMO channels.
Meanwhile, communication quality levels of the MIMO channels are not equal to one another, and the MIMO channels mixedly contains MIMO channels having low signal-to-noise ratios (SNRs) and MIMO channels having high SNRs. The communication quality level of an i-th MIMO channel corresponds to the eigenvalue λi that represents the diagonal element of the diagonal matrix D.
In the case that the power allocation to the individual MIMO channels is optimized, a communication capacity larger than that in the case of equal power allocations to the individual MIMO channels can be acquired. (In particular, the communication capacity is enhanced by allocation of transmission powers by MIMO channels having high communication quality (i.e., having large eigenvalues λi), as will be described in detail below.)
For example, where two MIMO channels are prepared and the overall transmission power is assumed to be 1.0, the power is allocated by 0.7 and 0.3.
However, where the power allocation is thus varied in units of the spatially multiplexed channel, accurate demapping cannot be executed on the side of the receiver.
This is because the varied power allocations vary the magnitudes of the amplitudes of reception signal points in a constellation (signal space). In an ordinary case, in any mapping scheme of BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 16QAM (quadrature amplitude modulated), 64QAM, and 256QAM, the average of powers is normalized to one under the assumption that all the signal points in the constellation are used, and the signal points are mapped. However, when the power allocations are varied, it is not guaranteed that the average of powers is normalized to one.
In the case of, for example, the BPSK or QPSK, demapping can be performed only by reference to the negative-positive relations of the reception signal points in the constellation on the individual coordinate axes, so that it is not necessary to all time guarantee the average of powers to be one. In comparison, however, in the scheme such as 16QAM, a plurality of signal points are mapped within a single quadrant of the constellation. As such, accurate demapping cannot be guaranteed by reference to the negative-positive relations on the individual coordinate axes, so that the average of powers has to be guaranteed to be one. More specifically, when the power allocations have been varied, the reception signal points have to be returned to the original magnitudes of the amplitudes on the side of the receiver.